The invention relates to a process for the preparation of biuret-containing polyisocyanates by catalytic reaction of aliphatic and/or cycloaliphatic diisocyanates with a biuretizing agent, in which, in accordance with the invention, the biuretizing agent employed is water in finely disperse form and the reaction is carried out in the presence of OH-acidic compounds.
Biruet-containing aliphatic and cycloaliphatic polyisocyanates are employed, inter alia, in high-quality light- and weather-resistant two-pack polyurethane coating materials.
The preparation of polyisocyanates having a biuret structure has been known for a long time, and is customarily accomplished by reacting diisocyanates with a biuretizing agent such as, for example, water or water donors, at relatively high temperatures, and then separating off excess monomer by distillation in one or more stages.
Thus, when using water donors such as tert-butanol as biuretizing agent, with acid catalysis, water is generated in situ. For this to occur, however, high reaction temperatures of more than 140.degree. C. are usually necessary, and at these temperatures the reaction products undergo a yellow discoloration. For reasons connected with their performance, for example as clearcoats, however, it is desired that the products be colorless as far as possible. Furthermore, the use of a water-donating and generally OH-containing biuretizing agent promote the formation of by-products which are devoid of biuret structure. These by-products may likewise have effects which are adverse in terms of performance properties, or may impair the stability of the product on storage, or give rise to process-related problems.
For this reason, the use of water as a biuretizing agent would be ideal. The use of water without additives, however, gives rise to two grave disadvantages:
1. The formation of insoluble polyureas in the reaction cannot in general be avoided. PA0 2. The products obained have a poor stability on storage with respect to breakdown to the monomer, thereby rapidly exceeding the 0.5% limit value (the inclusion of which on the label is mandatory), especially in the course of storage at above room temperature.
Attempts have been made to circumvent these problems by a modified procedure. The patent literature contains a multiplicity of variant methods for the preparation of polyisocyanates with biuret structure which employ water as biuretizing agent.
For example, U.S. Pat. No. 4,028,392 has already described the reaction of isocyanates with water in the presence of hydrophilic organic solvents such as, for example, trialkyl phosphates and ethylene glycol monomethyl acetate. According to EP-B-259 233, the reaction is carried out in the presence of at least one carboxylic acid and/or one carboxylic anhydride. Also envisaged is the concomitant use of methyl and/or ethyl esters of phosphoric acid, and of alkoxyalkyl carboxylates, as solubilizers. A disadvantage in this context is that the use of a solvent or solvent mixture in the necessary quantities results in a lower space-time yield of polyisocyanate being achieved than is the case without the use of solvent. In addition, a more complex distillation is necessary, possibly with the separation of solvent.
In EP-B-251 952, organic polyisocyanates with biuret structure are prepared from diisocyanates and water under a total pressure of at least 1.2 bar, at a partial pressure of carbon dioxide of at least 0.2 bar. However, this mode of operation using carbon dioxide at elevated pressure necessitates specific conditions and is therefore very complex to manage.
DE-C-29 18 739 describes a process for the preparation of polyisocyanates with biuret structure by reacting hexamethylene diisocyanate (HDI) with water, in which process the water is supplied in the form of vapor in a mixture with air and/or an inert gas, which mixture has a moisture content of from 0.1 to 2.0 kg/kg, to a reaction mixture which is maintained at from 110.degree. to 130 C., and reaction is carried out at from 150.degree. to 170.degree. C. The disadvantage here is that numerous insoluble polyureas are formed in the course of this reaction. This may lead to the clogging of plant components, especially lines.